Equations of State in Fighter Aircraft Oleo-pneumatic Shock Absorber Modelling

Most of all modern commercial and military aircraft have oleo-pneumatic shock absorbers in their landing gear. An oleo-pneumatic shock absorber consists of a gas charge and an oil fill. During the stroke oil is forced through orifices which provides damping, while the gas charge is compressed and acts as a spring by increasing the stiffness of the shock absorber. Typically, when the gas behaviour is modelled, the ideal gas law is used as the equation of state as this provides in most cases adequate fidelity with relatively light computational load. However, in a fighter aircraft, especially in naval service, the gas pressure inside a shock absorber raises too high during landing for the ideal gas assumption to be valid. Therefore, other well-established equations of state have been considered. These are Van der Waals, Redlich-Kwong-Soave, and Peng-Robinson equation of state. This paper presents a multi-physics simulation model of a two-chamber oleo-pneumatic shock absorber based on fundamental analytical equations. Using this model, the behaviour of the aforementioned equations of state are studied in two cases: quasi-static and dynamical compression. The simulation results are compared to laboratory measurements. This comparison verifies that the ideal gas law should not be used when modelling naval fighter aircraft shock absorbers.